Scale types

contain sets the scaled image to maximum have a width/height of the scale box. Always respects ratio.

cover sets the scaled image to at least have one of the width/height within the scale box. Always respects ratio.

fixed sets the scaled image to precisely the given width/height of the scale box. If both width/height is given it does not respect the ratio.

PDF Conversion

If you install ghostscript as well you will be able to convert pdfs to images by simply piping in a pdf and setting output format to jpeg (or another image format).

If you are rendering a multipage pdf scale.height will set the height of each page. To force scale.height to donate the height of the entire image set scale.multipage = true.

Use split = true to output each page as an image file. This will result in a tar file containing all the images, so you will need to untar them on the other end. Use a project like tar-stream to achieve this.